library: five protocols speak the envelope

The folded header stops being a rule in a document and becomes the layout
on the wire. Verbs number from sixteen, leaving describe, enumerate,
subscribe and unsubscribe reserved and answered the same way by every
provider — none of them writes a line to do it. What each protocol used to
carry in a field of its own now travels in the header: a vfs node and a
display layer are the packet's target, and a reply opens with a status the
envelope stamps rather than one each protocol spelled for itself.

Display gains the most. One forty-byte request had served eleven verbs, so
attach_scanout smuggled stride through x, refresh through y and format
through colour, and every coordinate crossed as a bitcast. Per-operation
structs end all three: the fields have their own names and their own signs,
and the tile payload grows to 224 bytes because the prefix shrank. Scanout
loses a message maximum of 64 it had no business declaring — it answers
calls, and the floor for a call is 256 — and virtio-gpu stops hard-coding
that number at its harness.

Two changes are semantic rather than notational. A directory now ends at an
entry with no name, because the fixed part of a reply always travels and a
zero-length reply no longer exists to mean anything. And input joins the
service harness, the last loop in the tree that answered no ping and heard
no terminate; its subscriber table, its pruning and its fan-out are the
same code, and a shutdown now asks it to stop instead of killing it.

A new conformance case reads the registry's own listing and asks every
protocol it finds for its name, its version and its verb count, then offers
a verb nobody defines and requires -ENOSYS — the envelope's promise,
checked against providers rather than against itself. What it cannot reach
in that boot it names on the serial line instead of passing quietly.

Suite 110/110.

system/services/input/build.zig

@@ -9,7 +9,7 @@ pub fn build(b: *std.Build) void {
     const exe = build_support.userBinary(b, .{
         .name = "input",
         .root_source_file = b.path("input.zig"),
-        .imports = &.{ "channel", "input-client", "input-protocol", "ipc", "logging", "process", "service" },
+        .imports = &.{ "envelope", "input-protocol", "ipc", "logging", "process", "service" },
     });
     b.installArtifact(exe);
 }